The present invention relates to a biosensor, specifically a cholesterol sensor, capable of carrying out speedy, highly-sensitive, simple determination of a specific component in a sample.
A description will be given to an example of a conventional biosensor, in terms of a glucose sensor.
In a typical glucose sensor, an electrode system including at least a measurement electrode and a counter electrode is formed on an insulating base plate by a method such as screen printing, and an enzyme reaction layer including a hydrophilic polymer, oxidoreductase and an electron mediator is formed on the electrode system. As oxidoreductase used is glucose oxidase; as the electron mediator used is a metal complex, an organic compound or the like, such as potassium ferricyanide, ferrocene derivative or quinone derivative. A buffer is added to the enzyme reaction layer, if necessary.
When a sample solution containing a substrate is dropped onto the enzyme reaction layer in the glucose sensor, the enzyme reaction layer is dissolved to cause a reaction of the enzyme with the substrate. This reaction leads to reduction of the electron mediator. After completion of the enzyme reaction, a substrate concentration in the sample solution can be determined from a value of oxidation current which is generated when this reduced electron mediator is electrochemically oxidized.
Namely, in this type of glucose sensor, a reductant of the electron mediator generated as a result of the enzyme reaction is oxidized on the electrode, to determine a glucose concentration from the oxidation current value.
In theory, such a biosensor is applicable to measurement of diverse substances by using an enzyme whose substrate is an object to be measured. For example, when cholesterol oxidase or cholesterol dehydrogenase and cholesterol esterase are used as oxidoreductase, it is possible to measure a cholesterol value in a serum to be used as a diagnostic indicator in various medical institutions.
Because the enzyme reaction of cholesterol esterase proceeds very slowly, with an appropriate surfactant added thereto, activity of cholesterol esterase can be improved to reduce the time required for the entire reaction.
However, the surfactant included in the reaction system has an adverse effect on hemocytes, making it impossible to measure whole blood itself, as done in the glucose sensor. For this reason, there has been made a proposal that a filter portion is provided in the vicinity of an opening in a sample solution supply pathway for a prompt supply of only plasma with the hemocytes therein filtered, to the sensor.
Nevertheless, a typical enzyme reaction layer includes an easy-to-dissolve part and a hard-to dissolve part. The part along the edge of the sample solution supply pathway is easy to dissolve, whereas the central part thereof is hard to dissolve.
Since the sample solution having passed through the filter flows through the edge of the sample solution supply pathway by priority, it closes an air aperture on the termination side of the sample solution supply pathway before complete dissolution of the central part thereof, leaving bubbles in the central part. In such a case, there is a problem that the sample solution of an amount necessary for measurement is not introduced into the sample solution supply pathway, whereby the enzyme reaction does not proceed sufficiently.
Moreover, there is another problem that the bubbles cover the electrode to reduce the substantial reaction area of the electrode, resulting in a measurement error.
It is thus an object of the present invention to provide a biosensor improved such that plasma with hemocytes therein filtered promptly reaches the electrode system, in order to obviate the disadvantages thus described. Further, it is another object of the present invention to provide a cholesterol sensor with high-accuracy and excellent response, capable of measuring whole blood.
The present invention relates to a biosensor, comprising: an insulating base plate; an electrode system which is provided on the base plate and has a measurement electrode and a counter electrode; a reaction layer including at least oxidoreductase and an electron mediator; a sample solution supply pathway which includes the electrode system and the reaction layer and has an air aperture on the termination side thereof; a sample supply portion; and a filter which is disposed between the sample solution supply pathway and the sample supply portion and filters hemocytes, where plasma with the hemocytes therein filtered with the filter is sucked into the sample solution supply pathway due to capillarity, and is characterized in that the central part of a secondary side portion of the filter protrudes into the sample solution supply pathway more than both the right and left ends thereof.
It is preferable that the secondary side portion of the filter is arc-shaped or semicircular in the projection thereof drawing to the plane face of the base plate which is the same as the surface thereof.
It is preferable that the sample solution supply pathway has a width of not more than 2.0 mm and the electrode system portion of the sample solution supply pathway has a height of not more than 0.2 mm.
It is also preferable that the biosensor has pressure portions for holding the primary side portion of the filter from the upper and lower sides, and the distance therebetween is not more than 1.5 mm.
It is also preferable that the biosensor has pressure portions for holding the secondary side portion of the filter from the upper and lower sides, and the distance therebetween is not more than 1.5 mm.
It is preferable that the primary side portion of the filter has a width of not more than 5.0 mm.
It is also preferable that the biosensor comprises at the opening in the sample solution supply pathway a concave portion fitted with the upper part or the lower part of the secondary side portion of the filter.
It is also preferable that in the biosensor, the cross sectional area of the sample solution supply pathway is smaller than the cross sectional area of the primary side portion of the filter.
It is also preferable that in the biosensor, the cross sectional area of the secondary side portion of the filter is smaller than the cross sectional area of the primary side portion thereof.